...the process to go from a customer-supplied DWG file, to finished product?

Or from customer-supplied DWG file(s), and STL file(s).

Is there that much variability in DWG files, that one person/firm's software can make it, specifically for CNC router production... but another shop can't open the file at all, or would have to completely re-create it from scratch in their software because of "something weird" in the file?

The couple projects that I've talked with local shops about were not overly complex (I think, might be wrong) - a router jig base plate with about 20 holes of various sizes, and an MFT type table top (MFSlab).